Setup basic functional tests and CI pipelines

[chadhakunal]

Set up initial functional tests that can be run in CI. End to end tests will be useful to ensure that things like load balancing, rate limiting, or request/response filtering are still working correctly with every release

• This would involve writing a configuration file for a certain scenario, running River, and making requests against it from another binary, such as curl or a Rust HTTP client.
• This would require setting up other servers for River to proxy to, which could either be more instances of River (serving static files, for example), or another HTTP server like NGINX or a Rust binary.